Electronically adjustable attenuators exhibiting discrete attenuation states have been implemented, for example, in “pi”, “T”, and “bridge-T” topologies. In each of these topologies, resistive elements are varied to produce a desired amount of attenuation, and each of these topologies may exhibit good return loss (at the input and output terminals) and desired attenuation between input and output when resistor values are properly selected. Typically, each of the “pi” and “T” topologies include three variable resistive elements, which are adjusted to produce a desired amount of attenuation. In contrast, the “bridge-T” topology includes only two resistive elements that need be varied to produce a desired amount of attenuation.
FIG. 1 illustrates a simplified schematic diagram of a conventional bridge-T type attenuator 100, which includes two variable resistive elements 110, 150 and two fixed resistive elements 130, 140. In attenuator 100, a first variable resistive element 110 is coupled between input and output terminals 102, 104. The two fixed resistive elements 130, 140 are coupled between the input and output terminals 102, 104, respectively, and an intermediate node. Finally, a second variable resistive element 150 is coupled between the intermediate node and a voltage reference terminal (e.g., ground). By varying resistive elements 110, 150, attenuation of a signal presented at the input terminal 102 may be adjusted to produce a signal at the output terminal 104 with a desired amount of attenuation.
Of particular interest with regard to implementing an attenuator topology at radio frequency (RF) frequencies is the choice and method of implementing the variable resistive elements. For example, using one technique, the variable resistive elements are implemented using PIN diodes (i.e., diodes including an intrinsic semiconductor region sandwiched between a P-type semiconductor region and an N-type semiconductor region), which have resistances that are inversely proportional to their bias currents. Although a topology using PIN diodes may have acceptable return loss and attenuation, a disadvantage to this approach is that significant DC current is consumed by each PIN diode. Accordingly, attenuator topologies for RF applications are desired, which exhibit good return loss and attenuation, while avoiding consumption of significant DC current.